Photovoltaic energy generation was the fastest growing energy source in 2007. In 2008, installed photovoltaic capacity increased approximately ⅔ to about 15 GW. By some estimates, the global market for photovoltaic power will grow at a compound annual rate of 32% between 2008 and 2013, reaching over 22 GW, while installed capacity grows at an average rate of 20-30% per year or more, possibly reaching 35 GW by 2013. With available solar resources estimated at 120,000 TW, using less than 0.013% of these available resources could replace fossil fuels and nuclear energy as sources of electrical power. Total global energy consumption of 16 TW in 2005 is less than 0.02% of available solar energy incident on the earth.
With so much potential, countries and companies around the world are racing to increase efficiency, and lower the cost of, photovoltaic power generation. In a typical solar cell, a semiconductor material is exposed to sunlight to mobilize electrons. Some portions of the semiconductor material are doped with electron-rich elements, and other portions are doped with electron-deficient elements to provide a driving force for the mobilized electrons to flow toward current collectors. The electrons flow from the current collectors out to an external circuit to provide electrical power.
The crystal structure of the semiconductor material influences the light absorption characteristics of the cell and the efficiency with which it converts light into electricity. In an amorphous semiconductor material, there are few straight paths for electrons to travel, so electron mobility is less, and the energy required to render the electrons mobile is higher. Amorphous silicon materials, thus, have a larger band gap and absorb light that has a shorter wavelength than light absorbed by a crystalline silicon material. Microcrystalline materials or nanocrystalline materials have some crystal structure, which gives rise to higher electron mobility on average, and lower band gap. Polycrystalline and monocrystalline materials have even higher mobility and lower band gap.
While it is desirable to include absorbers having different morphologies to capture more of the incident spectrum, only small amounts of, for example, amorphous materials are needed to provide the absorbance benefit. Too much amorphous material results in lower efficiency because electrons travel comparatively slowly through the amorphous material, losing energy as they go. As they lose energy, they become vulnerable to Shockley-Read-Hall recombination, falling out of the conduction band back into the valence of an atom, recombining with a “hole”, or local electron deficiency, and losing the absorbed solar energy that mobilized them.
To reduce this effect, it is thus desirable to maximize the polycrystalline and monocrystalline morphologies in a solar cell using a thermal treatment. Commonly used processes for treating deposited films and layers scan a line image of laser light across the solar substrate. The line image may be a few centimeters long and a few millimeters wide, so the image must be scanned across the substrate dozens of times to cover the entire area. Such scanning may take up to an hour to process each panel. The comparatively slow production rates require a large financial investment for a given productive capacity, driving up the cost of producing efficient solar cells and panels.
Thus, there is a need for improved apparatus and methods for manufacturing polycrystalline and monocrystalline semiconductor phases efficiently and at high rates.